FIG. 7A shows a fastener 7 of the one side type which comprises a bolt 71 having at one end a tip 73 to be sheared, a tube 72 fitting around the bolt 71, and a washer 75 and a nut 74 which are mounted the bolt 71.
Members 9 are fastened together by inserting the fastener 7 through a bolt hole 91 in the members 9 from one side thereof (see FIG. 7B), tightening the nut 74 at the same side to plastically deform the tube end at the other side into a flange 70 (see FIG. 7C) and further tightening up the nut 74 with the members 9 held between the flange 70 and the nut 74 (see FIG. 7D).
Next, the reaction of rotation of the nut 74 is caused to act on the tip 73 at the bolt end to shear the tip (see FIG. 7E).
The series of tightening steps described can be performed by a conventional shear bolt tightening device.
The tightening device comprises a planetary gear reduction mechanism having two output shafts coupled to a motor and rotatable in directions opposite to each other. One of the outputs shafts is connected to an inner socket, and the other output shaft to an outer socket.
The fastener 7 is tightened with the nut 74 engaged in the outer socket and with the tip 73 engaged in the inner socket.
The bolt hole 91 in the members 9 is so sized that the fastener is fittable therein with a small clearance formed in the hole. Accordingly, the bolt 1 is freely rotatable if a torque acts thereon until the nut 74 is tightened to such an extent that the end of the tube 72 is deformed into the flange 70 as shown in FIG. 7C or 7D to immovably fix the bolt 71 to the members 9. When the bolt is in this state as tightened by the conventional shear bolt tightening device, the planetary gear reduction mechanism will rotate in its entirety with the same speed of rotation as the input shaft of the mechanism, failing to tighten up the nut and presenting a hazard.
This problem can be obviated by incorporating into the device means for preventing the rotation of the inner socket. When the tip 73 is to be sheared in this case by causing the reaction of tightening the nut 74 to act on the tip 73 in fully tightening up the nut as shown in FIGS. 7D and 7E, the reaction will act on the housing, turning the entire device in a direction opposite to the nut tightening direction to pose a serious hazard.
Accordingly, a tightening device has been proposed wherein a restraint mechanism provided between the housing and a planetary gear support frame connected to the inner socket is disengageable by a predetermined torque, such that the entire device is prevented from turning in the opposite direction to the nut tightening direction when the nut is to be tightened up completely (Pre-examination Japanese Utility Model Publication SHO 62-95879). This mechanism comprises an engaging ball rollably inserted in a through hole in the housing and biased by a spring so as to partially project from the inner surface of the housing and usually engage in a cavity in an end face of the inner gear of the planetary gear mechanism, the ball being releasable from the cavity when the inner gear is subjected to a torque not lower than a predetermined value.
The single ball is in pressing contact with the end face of the inner gear, and therefore low in engaging force and reliability. Further since the ball is engaged in the cavity in the end face of the inner gear to hold the inner gear, the holding position is limited to the position where the cavity is opposed to the ball. Consequently, if the inner gear rotates on slipping, the nut will be subsequently tightened with the inner gear left disengaged from the ball, rendering the restraint mechanism unable to function as contemplated.
Further because the inner gear end face is formed with the cavity for the ball to engage in to hold the inner gear, the cavity portion will wear away if the inner gear repeatedly slips to rotate, failing to maintain a constant torque.
An object of the invention is to provide a bolt-nut tightening device free of the foregoing problems.